Boilers using demineralized or evaporated makeup water or pure condensate are known to be prone to caustic attack. High pressure boilers are particularly susceptible to this type of metal corrosion.
The inside surfaces of the boiler are typically protected with magnetite. Hydroxide ion, being the predominant anion in high purity boiler water, can dissolve the magnetite when highly concentrated. Even though high purity water is being used, caustic (NaOH) can nonetheless become highly concentrated, primarily due to the presence of iron oxide deposits on radiant wall tubes. While the bulk water may contain only 5-10 ppm of caustic, it is quite possible to have localized caustic concentrations of up to 100,000 ppm. The iron oxide deposits are generally porous so that the water is drawn into the porous deposit. Due to heat being applied from beneath, steam is generated and passes out of the porous deposit, while fresh water is again drawn into the porous deposit. The result is the noted high concentration of caustic which must be dealt with if the boiler is to be properly protected.
A widely used method for controlling caustic corrosion in boilers using demineralized (high purity) makeup water, particularly in high pressure boilers, is the coordinated phosphate/pH control treatment. This method of treatment is detailed in an article by George Gibson entitled "The Basics of Phosphate-pH Boiler Water Treatment", Power Engineering, February, 1978, page 66, which article is incorporated herein by reference to the extent necessary to complete this disclosure. In any event, portions are excerpted below for purposes of explanation.
The coordinated phosphate/pH corrosion control treatment is based on two principles:
First, that sodium phosphates are a pH buffer; and second, that disodium hydrogen phosphate converts potentially corrosive caustic into relatively harmless trisodium phosphate according to equation 1 below: ##STR1##
Accordingly, general corrosion is prevented through the control of boiler water pH. Adherent deposits with concomitant caustic corrosion are prevented by maintaining a disodium phosphate residual in the boiler water to react with caustic according to Equation 1 above.
In the program is implemented with a control chart such as shown in FIG. 1. Disodium hydrogen phosphate is present if the coordinate of pH and phosphate lies within the control boundary.
Many sodium phosphates are used in boiler water treatment. Of these, orthophosphates are preferred. Complex phosphates, in the form of polymer chains, breakdown into orthophosphates at boiler water temperatures by a process known as reversion. The orthophosphates are monosodium dihydrogen phosphate (MSP), disodium hydrogen phosphate (DSP) and trisodium phosphate (TSP).
Orthophosphates can be identified by name, formula, or sodium-to-phosphate molar ratio which can be expressed with the notation Na:PO.sub.4.
Monosodium dihydrogen phosphate has one mole of sodium per mole of phosphate. Therefore, the sodium-to-phosphate ratio is 1 to 1 (Na:PO.sub.4 =1:1). Disodium hydrogen phosphate, with two moles of sodium per mole of phosphate has a Na:PO.sub.4 =2:1, and trisodium phosphate has a Na:PO.sub.4 =3:1.
Sodium to phosphate molar ratios are useful to describe mixtures of phosphates in solution. For example, solutions of mixtures of DSP and TSP have a Na:PO.sub.4 between 2:1 and 3:1. The Na:PO.sub.4 is fairly proportional to the mix ratio. For instance, a solution of half DSP and half TSP has Na:PO.sub.4 molar ratio of about 2.5:1 (it is actually 2.46:1 because DSP and TSP have different molecular weights).
As shown in FIG. 2, the pH increases with increasing an Na:PO.sub.4 molar ratio (at equal phosphate concentrations). Accordingly, the solution pH and phosphate concentration identify the phosphate form, it being kept in mind that disodium hydrogen phosphate is the species which neutralizes caustic according to Equation 1.
A trisodium phosphate solution exists if the phosphate/pH coordinate falls on the Na:PO.sub.4 =3:1 line; a disodium hydrogen phosphate solution exists if the coordinate falls on the Na:PO.sub.4 =2:1 line; and a mixture of DSP and TSP exists if the coordinate falls between the 2:1 and the 3:1 lines. As the coordinate approaches the 3:1 line, there is more and more TSP and less and less DSP in the solution.
The solution is a mixture of TSP and caustic if the coordinate falls above the 3:1 line. In this "free caustic" region there is no DSP to tie up the caustic and caustic corrosion can occur.
In order for a coordinated phosphate/pH program to be successful, it is necessary to insure that a sufficient quantity of DSP is maintained to neutralize excess caustic. This is accomplished by monitoring the pH and phosphate level and using a control chart as shown in FIG. 1. If the coordinate of pH and phosphate lies within the control boundary, sufficient DSP is present.
There has been some confusion in applying sodium to phosphate ratios. The Na:PO.sub.4 used in phosphate/pH control is determined only from boiler water pH and phosphate concentration, not by measuring sodium and phosphate concentrations of the boiler water.
Problems can be encountered in controlling these programs due to the phenomenon known as "phosphate hideout" which occurs when elevated temperatures at the tube wall or beneath deposits induce precipitation because of retrograde solubility of certain salts. Hideout is usually observed when boiler load suddenly increases. The increase in boiler load is accompanied by a decrease in phosphate levels in the blowdown. When the load decreases, the phosphate level rises. This will be reflected on the control chart by showing that the system parameters lie outside the control boundaries. If precipitation increases the Na:PO.sub.4 above 3:1, caustic corrosion is possible.
Caustic corrosion requires high caustic concentrations which are not usually present in bulk boiler water, but may be found in areas where boiler water concentrates. This often occurs in porous iron deposits when water diffuses into the deposit, becomes trapped and boils. Boiling produces relatively pure steam which diffuses out of the deposit and leaves a concentrated caustic residue behind. Caustic leakages will also cause a system being treated with a coordinated phosphate/pH program to be "out of control". In order to bring the system back in control, the system must be blown down and/or additional phosphate must be added.
When a system treated with a coordinated phosphate/pH program is "out of control", caustic corrosion can occur. An object of this invention is to mitigate corrosion in a system during "out of control" periods. As used herein, the term "out of control" means that the system parameters, viz., phosphate concentrations, sodium-to-phosphate ratios, and pH lie outside the control boundaries of a control chart similar to FIG. 1.
While the coordinated phosphate/pH corrosion control treatment is widely used, it is not without its drawbacks or limitations. Often, it is desirable to supplement the treatment with additional corrosion inhibitor; however, this is not always practicable. It has been customary for many years to use the sodium salt of a polymeric dispersant, such as sodium polymethacrylate, as the supplement. When the sodium salt form is used, the Na:PO.sub.4 in the boiler water is often significantly altered and the solids level of the boiler water rises. If the Na:PO.sub.4 is allowed to rise over the 3:1 line of FIG. 1, caustic attack again becomes a problem, and, particularly in high pressure boiler systems, increased solids levels can lead to undesirable filming in the water. Thus, the use of supplemental treatment has been severely limited. In fact, when the Na:PO.sub.4 is near the control limit, the supplemental treatment has, on occasion, been completely omitted.
European Patent 0,018,083, published Feb. 2, 1985, discloses the use, in conjunction with a coordinated phosphate/pH corrosion control treatment, of an aqueous solution of an organic acid dispersant which has been neutralized with a suitable amine (or NH.sub.3) which is volatile under the conditions of the boiler water to be treated and has a basicity constant of 8.0 or less. The patent teaches the use of morpholine as a suitable amine for the purpose of neutralizing the organic acid dispersant.